It is a common misunderstanding that the Perpetual GMT's movement is a Lémania movement, or based on a Lémania. The latter is partially true, but only partially.
While the official "propaganda" of many watch manufacturers tries to suggest that the development of new movements and the production of watches is done in a typical Swiss way, that means, isolated, concentrated within the own four walls, redundance at its best, the truth is different - always has been.
Just like any other industrial production economy, it is common also in the watch manufacturing business, to team up with other companies, in order to share development and/or production costs. This is the reason why so many independent watchmakers are hired as experts for the development of some special complications, like tourbillons or repeater mechanisms. Illustrious names like Gerald Genta, Christophe Claret and many others would not be as illustrious without their background as contracted developers for high-end watch brands, even if those often choose not to announce the details of this cooperation publicly.
Another possibility of sharing resources would be that several companies team up to develop a part or movement that satisfies the needs of all of them. What is a standard procedure in the car industry (just think of the common platforms of Porsche's Cayenne and VW's Touareg), is also not uncommon in the watch production.
An example of this can be found in the movement of Ulysse Nardin's Perpetual Ludwig, and it's successor, the GMT Perpetual, and here is the story of how the caliber UN-32 was born:
Starting in 1973, Lémania produced a nice lever-operated automatic chronograph movement, the cal. 1340. This had a central 60 minutes-counter, similar to the display of the much cheaper Lémania 5100 movement. This movement had been produced in only small numbers (today watches with it are highly sought after, the most famous being the Omega Speedmaster 125), then the plans were stored in a drawer.
In 1989, Ebel was searching for a good replacement of the Zenith cal. 400 in its chronographs. As we all know, it was Ebel that kept the famous "El Primero" alive. However, after several years, Zenith needed the production capacity for itself, and Ebel sought an alternative.
During that time, Ulysse Nardin, too, was searching a base movement for Ludwig Oechslin's newly developed perpetual mechanism. Taking a selfwinding chrono movement as a base, would have the advantage, that the winding mechanism delivers enough power even for complicated mechanisms, but unlike a smaller base movement, it would be possible to integrate the perpetual mechanisms more densely into the base, with the parts responsible for the chrono operation been removed.
With such common need, it was an easy decision for Ebel and UN to join in their search for a good movement. This was found in the old Lémania design, and the rights to use it as a base were purchased. At that time, the Nouvelle Lémania, as it was named then, was still an independent company.
The main development effort on the new movement(s) was done in the former Ebel/Cartier design bureau, CEC 2. There, together with the experts from Ebel and UN, Lucas Humair, who is now responsible for the development of inhouse movements in UN's own R&D department, developed a completely new ebauche, which had only few design elements in common with the old Lémania 1340. A real novelty of the new movement was the unique way of using an eccentric mechanism to allow the rotor to wind the mainspring bidirectionally. Besides being very efficient, this mechanism also allows a thinner construction. While Albert Pellaton already used the same principle in the famous IWC cal. 654x selfwinding movements, the Ebel 137 "Le Modulor" (later, this designation was prohibited by Le Corbusier's heirs) was world's first chronograph movement with this mechanism. UN, too, adopted the principle of the winding mechanism, although it had to be completely reconstructed due to the different layout of bridges and wheel locations.
Thus, a movement family was developed, with the chronograph movement (now without the central minutes counter hand, but in the same classic dial layout as the Zenith cal. 400 - apparently this was a condition asked for by the Ebel side of the group), and the perpetual movement calendar - two entirely different complications, based on the same development.
As a result of this cooperation, each partner of the development group is the owner of its movement, as it is an own development. Each partner has its own set of plans and the right to use them however they choose to.
When the development was finished, a subcontractor for the production of many of the base parts was sought, with the Nouvelle Lémania being a logical choice. Lémania was also granted the right to use the chronograph construction for itself, to equip watches from other parties, just like Bréguet's Type XX. It can also decide to stop delivering the movement to other brands (Rainer Brand, for example). Ebel is not affected by this, and can continue to use its chronograph movement (Ebel 137) without interference by whomever the Lémania company actually belongs to. Of course, Lémania (now redesignated Manufacture Bréguet - I believe, this is the correct name) can stop producing the parts for Ebel, but cannot prevent Ebel from having them produced elsewhere. However, with Zenith in the same group, there might come the return of the "El Primero" in Ebel chronos, who knows?
And finally, there is Ulysse Nardin: UN is the owner of all movement plans for the perpetual and its design. It would be no problem to commission any other company with the production of parts, or - if the capacities are present - to produce all of them themselves, in the own UN production facilities.
Therefore, UN is fully independent from Lémania in regard of the Perpetual's movement, which is reason why the UN officials could react rather stoic on Lémania's widely debated announcement.
Some interesting figures will certainly straighten the proportions: The GMT Perpetual's movement, the cal. UN-32, consists of more than 200 parts. What is barely known, is that only 52 of them are supplied by Lémania as a kit, which is far from being a "base movement". Of these 52, Lémania produces only 28 in their own factory, according Ebel/UN drawings, while 24 are again bought by Lémania from other suppliers, also according to the given plans. The latter is an equally unknown, but also very interesting fact. All other parts for the UN-32, are either produced by UN in-house, or are subcontracted to specialist suppliers, who produce them following the UN drawings.
Hope this helps a bit,